1. Field of the Invention
The present invention relates to color processing for estimating illuminant information of an illuminant.
2. Description of the Related Art
As a process for estimating illuminant information of an illuminant represented by a fluorescent lamp, a process using a spectroradiometer is generally available. This is a process for estimating the illuminant information by measuring the spectral characteristics of an illuminant using a spectroradiometer and analyzing it. When an illuminant is a fluorescent lamp, its illuminant information can be obtained from a light color symbol described in the model of the fluorescent lamp. In addition, there is available a technique of restoring the spectral reflectance of an object from a sensor signal of an image capturing unit such as a digital still camera and estimating illuminant information using anomalous reflection (for example, see Japanese Patent Laid-Open No. 2004-177396).
Illuminant information will be defined herein. The color temperature and spectral type of an illuminant are referred to as illuminant information, hereinafter. Note that a spectral type represents the type of distribution of light emission wavelengths of a fluorescent lamp, that is, a broad-band type, three-band type, and normal type.
To obtain illuminant information using a spectroradiometer, purchase of illuminant measuring equipment is necessary. However, such equipment is expensive in general. For an end user of an image processing apparatus, it is difficult to readily obtain illuminant information by measurement since purchase of equipment forms a barrier against it.
A method of obtaining illuminant information from the model of fluorescent lamp can obtain color temperatures set as spec values, but cannot obtain the color temperature of a fluorescent lamp that has degraded due to aging. In addition, lighting equipment which uses fluorescent lamps often have lampshades attached to them. Since they conceal the model of the fluorescent lamp from users, it is sometimes difficult to obtain illuminant information from the model. In addition, there is an indirect lighting type lighting equipment in which the light of a fluorescent lamp does not directly come outside due to a lampshade. A color and material differ in respective lampshades, and the color temperature of light reflected by or transmitted through a lampshade is sometimes different from the spec value of the fluorescent lamp.
A technique disclosed in Japanese Patent Laid-Open No. 2004-177396 (U.S. Pat. No. 7,206,072) requires an image capturing unit which has four sensors of R, G, B, and X. When an image capturing equipment having such an image capturing unit is not available, illuminant information cannot be obtained.